GPS (i.e. a Global Positioning System) is a satellite-based radio navigation, positioning and time transfer system that provides highly accurate navigational information on a continuous global basis to an unlimited number of properly-equipped users. Important aspects of the system are that GPS is unaffected by weather and it provides a worldwide common grid reference system that is based on an earth-fixed coordinate system. Nevertheless, despite these beneficial aspects, GPS is susceptible to system outages, and is subject to jamming and spoofing.
For flight missions that require accurate position identification and precise navigational information for an airborne platform (e.g. an aircraft), GPS is an extremely effective tool. Due to the susceptibilities noted above, however, GPS should not be relied upon as a stand-alone navigational system. Stated differently, redundancy is a desirable attribute for any airborne navigational system. With this in mind, it would be desirable for a back-up navigational system for GPS to have certain features. First, the back-up system should be able to effectively assume the role of the primary system (i.e., GPS) when the latter becomes inoperative or inoperable. Second, and perhaps equally important, a back-up system can be used to verify the operation of the primary system.
Another important attribute for a back-up navigational system is that it be able to operate independently of the primary system it is intended to support. Preferably, such a back-up system not only operates independently of the primary system, it also relies on different physical phenomena for its functionality. This latter attribute is particularly important when jamming or spoofing of the primary system (e.g. GPS) is a distinct possibility.
With the above in mind, a passive system that allows accurate navigation using signals from pre-existing ground based emitters (e.g. television and FM radio broadcast signals, cellular phone system signals and signals from existing military equipment such as radar and communications transceivers) is an attractive candidate for use as a back-up to an airborne GPS system for several reasons. First, but not necessarily the most important reason, is cost. Because such a system contemplates the use of existing ground based emitters, this type of system could be implemented without incurring a large expenditure to establish and maintain emitters to produce navigation signals.
In addition to the intrinsic costs savings described above, a system having the ability to selectively use signals from a large number of diverse ground based emitters is relatively hard to jam. In particular, a system that can select among a population of emitters having a wide range of frequencies and waveform characteristics can continue to function despite the jamming or spoofing of a particular frequency band. In addition, a system having the ability to selectively use signals from a large number of geographically distributed ground based emitters is not easily rendered inoperable by a localized power outage.
Another advantage that is somewhat inherent in a navigation system that functions by receiving and processing signals from pre-existing ground based emitters is that the airborne platform is not necessarily required to transmit any-navigation signals. This can be important in certain applications, for example, when there is a concern that transmitted navigation signals might be used by hostile forces to locate, track and target the airborne platform.
In light of the above it is an object of the present invention to provide an on-board system for identifying the geolocation of an airborne platform that can serve as a relatively precise and accurate back-up navigational system for a GPS system. Another object of the present invention is to provide a passive system for identifying the geolocation of an airborne platform that can function as a stand-alone system. Still another object of the present invention is to provide a system for identifying the position of an airborne platform that is not easily jammed, spoofed or rendered inoperable by a localized power outage on the ground. Yet another object of the present invention is to provide a system for identifying the position of an airborne platform that is accurate, relatively inexpensive to implement and relatively simple to use.